1. Field of the Invention
The present invention refers to a high-pressure compression unit, preferably but not exclusively for use in re-injection plant for gases, whether acid or not, and a related method for compressing a process fluid.
2. Description of Related Art
A compressor is a machine capable of increasing the pressure of a compressible fluid (gas) through the use of mechanical energy. The various types of compressor used in process plant in the industrial field include so-called centrifugal compressors, in which energy is supplied to the gas in the form of centrifugal acceleration due to rotation, generally controlled by a driver (electric motor or steam turbine), through a component called a rotor or impeller.
Centrifugal compressors may be fitted with a single rotor, in the so-called single stage configuration, or may have a number of impellers arranged in series, also known as multistage compressors. More precisely, each of the stages of centrifugal compressor is normally composed of an intake duct for the gas to be compressed, an impeller, which is able to supply kinetic energy to the gas, and a diffuser, the role of which is to convert the kinetic energy of the gas coming out from the impeller into pressure energy.
Gas injection is normally the reintroduction of natural or inert gas into subterranean deposits of hydrocarbons, typically containing both gases and liquid crude oil, so as to increase the pressure within the deposit itself, improving the extraction capacity for crude oil, and therefore the yield of the well. In addition, the re-injection of gas, particularly acid gas, into the deposit, can contribute to a reduction in the environmental impact that would otherwise occur if it were necessary to dispose of the residues from treating the gas.
Hydrocarbons are organic compounds which contain atoms of carbon and hydrogen.
In short, in hydrocarbons, the carbon atoms (C) are linked to one another to form the core of the molecule, while the hydrogen atoms (H) extend from this core. Up to the present time, more than 130 thousand types of hydrocarbons have been classified. The most simple hydrocarbon is methane, having a formula CH4. Increasing the number of carbon atoms, gives ethane, with a formula C2H6, ethene (or ethylene), C2H4 and acetylene, C2H2. In particular, crude oil is composed of a mixture of various hydrocarbons, alkanes, but with differences in appearance, composition and physical/chemical properties. Hydrocarbons are present in nature in various forms and in mixtures with other gases, which are of little interest and which are difficult to dispose of.
In compression plants that carry out the re-injection of gas, which are becoming increasingly widespread in the oil and hydrocarbons industry, it is necessary to have compression units available that are capable of operating at high pressures, which at present are quantifiable from 100 bar to approximately 300 bar. Moreover, it is predicted that future applications will require compression units with higher performance, in order to compress the gas to pressures in excess of 500 bar.
In order to compress the fluid, without condensates, it is possible to compress it by limiting or eliminating inter-refrigeration, with a consequent reduction in the efficiency of the compression process itself.
Likewise, it is possible, once the critical state of the fluid has been reached by means of compression, to condense it through cooling and to continue the compression by means of a pump positioned externally with respect to the compression unit itself.
One disadvantage of the traditional high-pressure compression units is the fact that they are technically difficult to design because of the various problems of a mechanical or fluid-dynamic nature that are encountered on increasing the maximum output pressure. Examples of such technical difficulties are: the complications of the systems of external sealing, the fluid dynamic performance and others.
Another disadvantage is that the compression units are increasingly required to operate at pressures well above the critical pressure of the process fluid, causing a worsening of the above-mentioned technical problems. In addition, the compression of a super-critical fluid at high temperature reduces the efficiency of the compressor.
A further disadvantage is that in the event that a normal pump is used externally to the compression unit, even though such use may contribute to a significant increase in the cost of the plant, there is a high risk that losses of gas into the atmosphere will arise, which is particularly critical if acid gases are present.
In fact, the use of a pump mechanically connected to the compression unit by means of a shaft passing to the outside, although in some cases this may reduce the mechanical complexity of the machine (it is possible to use a single motor to drive the compressor and the pump), it does bring a significant risk of gas losses from the external dynamic seals that must be fitted on the shaft connecting the unit and the pump.
These external dynamic seals are therefore particularly critical in the presence of acid fluids, which increases the cost of design and maintenance of the unit in order to guarantee the necessary safety.
A further disadvantage is the fact that traditional machines are bulky and heavy and therefore relatively expensive to transport and install, particularly in marine or submarine applications where weight is important, such as for example in platforms, “Floating Storage and Offloading units” (units operating at anchor in the open sea for the storage of oil after extraction from a marine field), submarine wells and other cases.
Therefore at present, in spite of the developments in technology, problems remain and a need is recognized for the production of a high-pressure compression unit for fluids, particularly but not only acid or dangerous gases, which has a higher performance, is economically sustainable both in its construction and in maintenance, and which at the same time guarantees a reduction of risk of losses to the external environment.